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From Galápagos albatrosses to polar bears, primates to great apes, certain species appear to kiss. Currently, scientists propose that ancient hominins also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Shared Oral Clues

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were intimately acquainted. Among earlier research, researchers have found modern people and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were kissing," the researcher noted, adding that the idea chimed with research that has revealed humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing genetic mixing was occurring.

Romantic Interpretation

"It certainly puts a different perspective on ancient interactions," the lead researcher commented.

Writing in the journal Evolution and Human Behavior, the researcher and her team detail how, to explore the historical roots of kissing, they first had to come up with a definition that was not restricted by how humans smooch.

Defining Intimate Contact

"There have been some previous attempts to define a intimate act, but it's largely human-centric, which means that basically other animals do not engage in this. Currently we know that they likely engage, it might just not look from what human kissing resembles," said Brindle.

However, she said some actions that resembled kissing were something rather different – such as the processing and transfer of food, or "kiss-fighting", seen in aquatic species known as French grunts.

Consequently the research group developed a description of kissing centered around social behaviors involving intentional oral interaction with a member of the same species, with some movement of the oral area but no transfer of nutrition.

Research Methods

Brindle said they focused on accounts of kissing in non-human species from the African continent and Asian regions, including bonobos, apes and great apes, and used digital recordings to confirm the observations.

Scientists then integrated this information with details on the genetic connections between extant and extinct types of such primates.

Historical Timeline

The team propose the findings suggest kissing evolved somewhere between 21.5m and 16.9m years ago in the ancestors of the great primates.

The position of ancient hominins on this family tree means it is likely they, too, indulged in a intimate act, the scientists conclude. But the activity may not have been confined to their specific group.

"Reality that modern people kiss, the fact that we now have demonstrated that Neanderthals probably kissed, suggests that the both groups are also likely to have kissed," the researcher noted.

Evolutionary Significance

While the evolutionary explanation is debated, Brindle explained intimate contact could be employed in reproductive situations to possibly increase mating outcomes or help choose between mates, while it could assist strengthen connections when practiced in a non-sexual manner.

A separate researcher in the activities of great apes commented that as kissing behavior was observed in a broad spectrum of primates it was logical its roots lie deep in our evolutionary past, and an analysis of various types of intimate behavior among a wider variety of animals might push its beginnings back even earlier still.

"Things that we think of as characteristics of human life, like intimate contact, are not unique to us if we look closely at different species," he said.

Social Elements

Another professor explained that intimate contact had a cultural element as it was not common to all societies.

"Nonetheless, as people we succeed or struggle on the strength of our emotional bonds, and ways of encouraging trust and closeness will have been important for eons," the professor stated. "This could represent an image that seems a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but actually it ought to be no surprise that Neanderthals – and including them and our human ancestors collectively – kissed."